1. Field of the Invention
The present invention relates to a position measuring apparatus and a positional deviation measuring method and, more particularly, to an apparatus and a method for measuring a pattern positional deviation of EUV (Extreme Ultra Violet) masks written or “drawn” by using variably-shaped electron beams, for example.
2. Related Art
In recent years, circuit line widths of semiconductors are becoming narrower and narrower with an increase in pattern density. In order to precisely make large scale integrated circuits on a silicon (Si) wafer, exposure technique of transferring an original written on a mask (also called a master or “original” pattern or a reticle) is progressing. For example, technique of optical proximity effect correction of arranging sub-resolution assistant features, which are not to be printed, around an original mask pattern is developed. Alternatively, off-axis illumination technique of giving anisotropy to lights used for printing in order to partially increase resolution is developed. In addition, liquid immersion exposure technique of filling liquid, such as water or special oil having a refraction index larger than that of air, between an objective lens and a wafer to increase a resolution limit is also developed.
By virtue of these techniques, a pattern equal to or less than 90 nm, which is half of 193 nm being a wavelength of an exposure light source, is becoming producible. Particularly, in the liquid immersion exposure technique, it is shown that a pattern of 45 nm can also be printed based on a theoretical refraction index of water. Therefore, it is thought if still more ideal oil is found, a pattern of near 32 nm can be printable by utilizing the liquid immersion technique.
However, in such exposure technique, it is assumed that sub-resolution assistant features for correcting an optical proximity effect may become complicated. Although sub-resolution assistant features are patterned on a mask and not printed onto a wafer, it has an influence when an transferring image is printed on the wafer. The sub-resolution assistant features become complicated in proportion as the influence of an aerial image becomes large. Moreover, the complicated pattern has a large influence on writing time of an original mask. Furthermore, there is also a very big problem concerning the way to inspect sub-resolution assistant features on a mask.
In order to solve those problems mentioned above, shortening wavelength itself of an exposing light is also considered similar to the prior improvement techniques of lithography. Developing new lithography technique with a light of 157 nm has been given up due to lack of lens material for the optics used for image shrinking and transferring. For this reason, it is developed that the extreme ultraviolet (EUV) light with a wavelength of 13.4 nm becomes most promising at present. As to the EUV light, which is classified into a soft-X-ray area, it cannot make a projection optics any longer because it is penetrated/absorbed by all materials being discovered. Therefore, a catadioptric optics is proposed for the exposure system using the EUV light.
Concerning a technique to hold EUV masks, a method of chucking almost all the backside in a planar state is proposed instead of a conventional method of holding the circumference by three or four points in order to let transmitted lights pass. Furthermore, since the holding system itself of EUV mask is installed in a vacuum chamber in order to prevent attenuation of EUV light, it is premised on use of an electrostatic chuck, in order to hold a mask for EUV, called an EUV mask hereinafter. Guidelines for substrates to be exposed and electrostatic chucks themselves are severely defined as they are specified in the SEMI standard. Refer to, for example, “SEMI P38-1103 SPECIFICATION FOR ABSORBING FILM STACKS AND MULTILAYERS ON EXTREME ULTRAVIOLET LITHOGRAPHY MASK BLANKS”, “SEMI P37-1102 SPECIFICATION FOR EXTREME ULTRAVIOLET LITHOGRAPHY MASK SUBSTRATES”, or “SEMI P40-1103 SPECIFICATION FOR MOUNTING REQUIREMENTS AND ALIGNMENT REFERENCE LOCATIONS FOR EXTREME ULTRAVIOLET LITHOGRAPHY MASKS.”
Moreover, when fabricating a master EUV mask, it is difficult to predict total deformations of a substrate in the step of forming a reflective film, or in the process of patterning. Therefore, according to the above SEMI P40-1103, holding a substrate by an electrostatic chuck is essential for pattern writing apparatuses, position measuring apparatuses, and exposure apparatuses
FIG. 24 shows a schematic diagram for explaining operations of a conventional variable-shaped electron beam pattern writing apparatus. As shown in the figure, the variable-shaped electron beam pattern writing apparatus (EB (Electron Beam) writing apparatus) includes two aperture plates A first or “upper” aperture plate 410 has an opening or “hole” 411 in the shape of rectangle, for example, for shaping an electron beam 330. This shape of the rectangular opening may also be a square, a rhombus, a rhomboid, etc. A second or “lower” aperture plate 420 has a special shape of opening 421 for shaping the electron beam 330 having passed through the opening 411 of the first aperture plate 410 into a desired rectangular. The electron beam 330 that left a charged particle source 430 and has passed through the opening 411 of the first aperture plate 410 is deflected by a deflector. Then, the electron beam 330 passes through part of the special shape of opening 421 of the second aperture plate 420, and reaches a target workpiece 340 mounted on a stage which is continuously moving in one predetermined direction (e.g. X-axis direction). In other words, a rectangular shape capable of passing through both the opening 411 and the special shape of opening 421 is used for pattern writing of the target workpiece 340 mounted on the stage. This method of writing or “forming” a given variable shape by letting beams pass through both the opening 411 and the special shape of opening 421 is called the “variable shaping.”
It is also very difficult to meet the guidelines of electro-static chucks with precision and accuracy, described in the SEMI standard (SEMI P40-1103), and furthermore to check the chucks being met the specifications. Moreover, according to ITRS roadmap, the particle diameter of a particle permitted in the process of EUV mask making is 30 nm and/or less. As to the backside of an EUV mask, a conductive film, such as Cr having sufficient adhesion to glass, is coated on the backside for an electrostatic chuck. In the case of employing the method of the electrostatic chuck, etc. in which the area of the surface contacting with a mask is large, there is much possibility of the electric conduction film on the backside being damaged by friction etc. generated at a contact part and such damaged film becoming a particle. Moreover, if a particle exists on the backside of an EUV mask, there is concern to fail to meet the requirements of image positional accuracy of the pattern because of a local deformation of the EUV mask caused by the mask backside not having a tight contact in and around the particle. Therefore, it is necessary to always retain the chuck surface to be clean. However, to retain and manage the chuck surface to be such clean is very difficult.
Furthermore, since a mask is generally used as a master in the exposure apparatus so that images can be shrunk and transferred one by one onto a wafer, only the mask which has passed a final cleaning process is used. However, in case of making an EUV mask with the pattern writing apparatus, it is necessary to use the EUV substrate with resist which being a photosensitizing polymer is applied similar to the case of writing a pattern on an optical mask. As the optical mask described herein, the one used in an exposure process by utilizing lights other than EUV lights, for example, ultraviolet rays is mentioned. Similarly to the ordinary optical mask, the resist applied on the EUV mask acts as a photosensitizing polymer and causes a chemical reaction to the intended pattern written with electron beams. As a result of this, only the part which has changed in quality by the irradiation of the electron beams on the pattern is eliminated (positive type resist) or a part other than the irradiated part on the pattern is eliminated (negative type resist) in a next development process in order to obtain a resist pattern. Then, using the resist as a protective film, chromium (layer under the resist layer) is removed by etching in the case of an ordinary optical mask, or metal of chromium family or tantalum family being a shading film is removed by etching in the case of an EUV mask. Consequently, a mask which lets lights pass through only the removed part can be obtained. Then, the resist left as a protective film of etching is removed by chemically resist stripping processing.
Regarding this resist, it needs to be applied thinly and uniformly whether it is in the optical mask case or the EUV mask case. Generally, resist is composed of a polymer film whose main component is carbon, and is applied by a spin coating technique which trickles the resist, melted in solvent, of a predetermined amount on a spinning substrate. Although there is a possibility of the resist partially going around to the side and the backside of the substrate at the time of the application, it is very difficult to remove the residues and adhesive substances, such as resist, on the side or the backside without any influence on the resist of the mask surface. In addition, after the resist is coated, baking (prebaking) is performed at a predetermined temperature for mainly stabilizing and equalizing sensitivity based on the kind and conditions of the resist.
However, even when the baking process is performed, the resist being a polymer film has a feature of easily damaged and removed. When it is necessary to load a substrate or hold a substrate during writing in the pattern writing apparatus, only limited areas are touched to handle and hold in order to avoid any contact to the inside of limited areas on the mask surface. In this situation, it is easily considered that the resist which unexpectedly goes around to the side or the backside may become a cause of particulate contamination in the pattern writing apparatus because such resist removes or attaches to a contact portion.
Furthermore, when an electrostatic chuck is used for the EUV mask, since almost all the backside of the mask contacts with the chuck, it is much expected that adhesive substances remaining on the side or backside, such as resist, are removed to become particles, and consequently they are attracted by the chucking surface of electrostatic chuck. Therefore, it becomes difficult to retain the chuck surface in a clean condition. As a result, since the particles on the electrostatic chuck surface are contacted with the mask backside, keeping the mask backside to be an ideal plane becomes difficult.
Then, another method of writing a pattern is proposed in which the backside shape of a substrate being a mask is measured during writing or before writing, in a state of the substrate being held without using the electrostatic chuck, and a positional deviation of the pattern is calculated and corrected based on the measured backside shape of the substrate. (See JP-A-2004-214415, for example.
However, according to the technique disclosed in JP-A-2004-214415, height position distribution of the substrate backside, being opposite to the surface on which a pattern is written, is measured. Then, because of employing this method, it comes under the influence due to the gravitational sag at the time of measuring the substrate backside, and there is a possibility that a amount of the gravitational sag for each substrate may change depending upon a tolerance of the substrate thickness or a deformation amount may change, in each substrate, with the change of multilayer film stress peculiar to EUV masks. Therefore, a problem may arise in the reproducibility when reproducing, by calculation, a state of the substrate backside being corrected to be a desired curved or flat surface. As to a measuring device of height distribution, there is a measuring instrument utilizing an interferometer generally used to measure flatness of EUV masks. However, it is very difficult to equip this measuring instrument on the pattern writing apparatus because of structure restrictions of the apparatus. Accordingly, even if there is a measuring instrument which can be equipped on the pattern writing apparatus in consideration of the structure restrictions, there is concern that resolution of the measuring device may not enough.
Further, even if the pattern writing apparatus can perform writing, a problem occurs in a position measuring apparatus which measures a position of a written pattern. That is, since the mask to be measured by the position measuring apparatus is a substrate prior to the step of the final cleaning process, residues, such as resist, and particles adhered during processes may remain. If such EUV mask is held by the electrostatic chuck, particles on the substrate will be attracted by the electrostatic chuck. Therefore, it becomes difficult to retain the chuck surface in a clean state. Moreover, since highly precise measurement is required, the current position measuring apparatus needs to be installed in a homoiothermal chamber in which temperature and humidity are controlled in order to make environmental change small. Since there is no necessity of operating the position measuring apparatus in a vacuum at the time of pattern position measurement, it is desirable to use it in the atmosphere where handling and management of the apparatus is easily performed. However, if the electrostatic chuck is used in the atmosphere, there is a great risk of attracting positive-charged substances in the environment by the static electricity. Consequently, since particles enter between the electrostatic chuck surface and the mask backside, keeping the mask backside to be an ideal flat surface becomes difficult. Thus, there is a possibility that highly precise position measurement cannot be obtained.
As mentioned above, there is a problem that when using an electrostatic chuck for holding an EUV mask in the position measuring apparatus, making the electrostatic chuck which fulfills specifications in the guidelines advocated by SEMI is very difficult. Moreover, even if it becomes possible to use the electrostatic chuck which fulfills the specifications, there is a problem, such as a particle management, in the position measuring apparatus. In addition, even if it becomes possible to use the electrostatic chuck which fulfills the specifications, there is a problem stated below. That is, with respect to a pattern written by data correction without using the electrostatic chuck, it is impossible for the position measuring apparatus using the electrostatic chuck to evaluate image placement accuracy of the pattern writing apparatus that has written such pattern.